Electrical profiling of this type has been known for a long time and has been extensively used particularly in recent years as a tool for detailed description of the subsurface geology in connection with e.g. pollution. For this purpose there is usually used an electrode array in a so-called Wenner configuration with four electrodes in the form of earth rods, which are driven down into the earth in a row, with the same mutual spacing and the measuring point defined as the center point of the array. Then a constant electrical current is passed through the earth between the two outermost earth rods, the current electrodes, by means of a current generator, following which the voltage drop of the current across the two central earth rods, the voltage electrodes, is measured. Prior to taking this measurement, the electrochemical surface potentials must have stabilized to a constant value, and this typically takes 20-30 seconds. To balance this residual error contribution two or more measurements are taken in the same electrode array, the current direction being reversed each time, so that the error contribution is substantially balanced owing to the subtraction of the individual measurement results involved by this method. This technique is known e.g. from the patent specifications U.S. Pat. No. 2,640,097 and GB 2 106 653. On the basis of the given current strength, the measured voltage drop across the voltage electrodes and a factor depending solely upon the geometry of the array, the electrical resistance of the earth formation corresponding to a specific focusing depth is finally calculated.
A so-called line profiling is typically used in the taking of the measurements, the measuring configuration being successively moved along a line with the same spacing between the measuring points, and since the electrical resistance is frequently to be registered at several different focusing depths at the same measuring point, the rods are moved symmetrically around this measuring point, and then the measurements are repeated at various rod distances corresponding to the focusing depths to be measured.
When a large number of these line profilings is performed successively, a sum of measurement results can be provided which, in combination, provide a reliable mapping of the electrical resistance of the strata, and thereby give, when interpreted, a satisfactory picture of the geological structure of the area. As will appear, the method is extremely slow and cumbersome, and this applies especially when, as is often the case, it is necessary to examine the formation at several different focusing depths for each individual measuring point, and these are positioned relatively closely to each other for the area to be mapped with a sufficiently high level of detail.
With a view to facilitating this slow and laborious field procedure it has been proposed in U.S. Pat. No. 2,105,247 to use mobile electrodes in the form of e.g. endless tracks on a tractor travelling across the surface of the earth while measurements are continuously taken. In this method it was found necessary to use a relatively great current strength of between 5 and 20 amperes to obtain a voltage signal which was so powerful that it could be distinguished with reasonable certainty from the electrochemical surface voltages which occurred as a consequence of the passage of the voltage electrodes across the varying chemical environment of the surface of the earth. For such a great current strength to be transmitted, the electrode pressure against the earth had to be relatively great to form a sufficiently small transitional resistance between the electrodes and the earth. Thus, electrodes having a weight of between 50 and 100 kg per electrode have been used in practice, and these heavy and unhandy electrodes in connection with the high and dangerous current strengths have entailed that the proposed technique has only been used on a purely experimental basis.
Another form of mobile electrodes is described in the patent specification DD 145 671, the electrodes being constructed as a sort of knives which are secured under their respective carriages which are towed in a row by a towing means, e.g. a vehicle. When the electrodes are constructed as knives, the transitional resistance between the electrodes and the earth is reduced with respect to the method known from the above-mentioned U.S. Pat. No. 2,105,247. On the other hand, such electrodes can be used in practice only in homogeneous, relatively loose stata, because the knives cannot or only with difficulty cut through firm and hard formations, and because they lose contact with the earth or get stuck if they hit a stone. If the voltage signals are to be distinguished with reasonable certainty from the electrochemical voltages in the uppermost stratum using when only the means described in the patent it would also be necessary to use relatively great current strengths which constitute a safety hazard to the operators. Furthermore, the patent does not teach how to solve the above-mentioned problems, and how to make the measurement, and also fails to mention the specific electrical means which are used in the measuring system.
The U.S. Pat. No. 3,808,521 describes an instrument for measuring the electrical earth resistance. Unharmful current strengths below 10 mA are used in this case, so that in the described electrical circuit it is necessary to ensure extremely intimate contact between electrodes and earth to reduce the transitional resistance to such a small size as to provide an even tolerably satisfactory measurement result. Accordingly, this instrument cannot be used in connection with mobile electrodes.